8.14 Consider the reactions :

2 S2O32– (aq) + I2(s) S4 O6 2–(aq) + 2I(aq)

S2O3 2–(aq) + 2Br2(l) + 5 H2O(l) 2SO42–(aq) + 4Br(aq) + 10H+(aq)

Why does the same reductant, thiosulphate react differently with iodine and bromine ?

The average oxidising number (O.N.) of S in S2O32- is +2.

Being a stronger oxidizing agent than l2, Br2 oxidises, in which the O.N. of S is +6. However, l2 is a weak oxidising agent.

Therefore, it oxidises, in which the average O.N. S2O32- or S4O62- of S in only +2.5.

As a result, S2O32- reacts differently with iodine and bromine.